Forgot to mention, this was both on a device, and in the simulator.

Same here. I have a swiftData app, that was working fine. Upgraded to XCode 15.3 stable last night, and suddenly CPU usage is basically 100%, in the simulator (iOS 17.4) even when the app is just sitting idle. Downgraded to XCode 15.2, and the problem is gone. CPU usage ~1% when idle.